1. Field of the Invention
The present invention relates to an RCC (Ringing Choke Converter) type switching power supply in which a voltage error of a DC output is detected on the basis of a reference voltage generated by a Zener diode, and a video cassette recorder equipped with the RCC type switching power supply.
2. Description of the Related Art
A configuration in which a reference voltage stabilized by a Zener diode D11 is led to an emitter of a transistor Q11 as shown in FIG. 2 is used for detecting a voltage error of a DC output in a switching power supply or the like. In this case, the voltage range of a DC output 92 allowed to be subjected to error detection is limited to a voltage range higher than the sum of the Zener voltage and the base-emitter voltage of the transistor Q11 for detecting an error. On the other hand, good temperature characteristic can be obtained when temperature characteristic of the base-emitter voltage of the transistor Q11 and temperature characteristic of the Zener diode D11 cancel each other. However, when the voltage of the DC output 92 is set at 5 V, an element exhibiting a Zener voltage of not higher than 4.4 V needs to be used as the Zener diode D11. In this case, the temperature characteristic of the Zener diode D11 is minus. The temperature characteristic of the base-emitter voltage of the transistor Q11 is minus too. Accordingly, total temperature characteristic is poor. It is therefore necessary to use a shunt regulator as an IC for stabilizing a low voltage such as 5 V. This however causes increase in cost of parts because the shunt regulator as an IC is expensive.
A technique for eliminating the disadvantage has been already proposed by the applicant of the present application (see JP-A-2001-034350). In the proposed configuration, a voltage is stabilized by a Zener diode D15 connected to a DC output 93 of 12 V through a resistor R25 as shown in FIG. 3. The stabilized voltage is divided by resistors R26 and R27, so that a voltage at a junction between the resistors R26 and R27 is supplied to a base of a transistor Q25. On the other hand, a voltage of a DC output 94 as a subject of error detection is divided by a voltage divider circuit constituted by resistors R28 and R29, so that an output voltage of the voltage divider circuit is supplied to a base of a transistor Q26. A photo-coupler 95 for feeding a detected voltage error back to the primary side is connected to a collector of the transistor Q26. An emitter of the transistor Q25 and an emitter of the transistor Q26 are connected to each other. Accordingly, a temperature-dependent change in the base-emitter voltage of the transistor Q25 and a temperature-dependent change in the base-emitter voltage of the transistor Q26 cancel each other. As a result, when an element exhibiting good temperature characteristic at 5.6 V is used as the Zener diode D15, the voltage of the DC output 94 can be stabilized with good temperature characteristic if the voltage of the DC output 94 is a voltage (e.g., 5 V) lower than the Zener voltage of the Zener diode D15.
In the aforementioned configuration, the two transistors Q25 and Q26 are however essential for error detection because a temperature-dependent change in the base-emitter voltage of the transistor Q26 for detecting an error must be canceled by a temperature-dependent change in the base-emitter voltage of the transistor Q25. As a result, the number of transistors increases to bring increase in cost of parts.